The Centre for Innovation in Mathematics Teaching have online textbooks / notes available for students age 5 to 18!
For example see Year 7(scroll down to see the chapter headings for books 7A and 7B.
Scrolling down this page you will see the range of resources available including GCSE and A Level Notes. The interactive materials for Years 7, 8and 9are useful for self study; it may be worth checking all three years for the topic you want. Students starting Advanced level courses (age 16-18) may find the transition materials useful.

The HELM Project. If you have not come across the HELM Project before, the project was designed to support the mathematical education of engineering students and includes an extensive collection of notes which include clear worked examples. You can see on the list that a very small number of titles are ‘not ready yet'; for the sake of completeness the complete set is hosted by the Open University. To access the Open University resources you will need to create an account (easy and free), this will also give you access to the numerous free online courses.

See this Evernote shared notebook:Mathematics notes for many more useful links. Several universities have created very helpful Mathematics support which they have made available to all students. (You do not need an Evernote account to view the notebook).